Deworming Horses: Expert Recommendations to Avoid Resistance

From Kentucky Equine Research:

No new dewormers for horses are currently in development, and widespread resistance to existing products is on the upswing. As a result, global efforts to slow antiparasitic resistance are gaining momentum.

Julie Shrubb, a veterinary surgeon based in the United Kingdom who chairs Controlling Antiparasitic Resistance in Equines Responsibly (CANTER), an initiative promoting sustainable parasite management, offers several key recommendations.

In a recently published article, Shrubb notes, “It is no longer acceptable to give any dewormer without risk assessment and/or testing.”^*

Testing and Thresholds

A fecal egg count (FEC) is recommended to identify high shedders. Targeted deworming of this specific population reduces pasture contamination and decreases parasite transmission across the herd.

“The standard ‘threshold to treat’ is 200 to 250 eggs per gram of feces, but some experts now recommend a threshold of 500 eggs per gram for horses kept in low- to medium-risk environments,” noted Kathleen Crandell, Ph.D., a nutritionist at Kentucky Equine Research.

Assessing Risk Factors

Risk assessment is the second critical component of responsible deworming. Horses in low-risk environments, which are less likely to require frequent treatment, typically share these characteristics:

• Low FEC history: Repeatedly low or negative egg counts in individual horses and herds.
• Age range: Adult horses between 5 and 20 years old.
• Pasture management: Routine removal of manure from pastures.
• Stocking density: Low density of horses per acre.
• Biosecurity: Limited movement of horses in and out of the herd, along with strict quarantine protocols for new arrivals.
• Health history: No history of parasitic colic, parasite-related disease, or reported anthelmintic resistance.

“Veterinarians play a crucial role in educating owners about responsible deworming, and owners are encouraged to be receptive to these discussions during routine health exams,” Crandell said.

Looking Ahead

Alternatives to chemical dewormers, including specific bacteria, fungi, and herbal formulations, are currently being explored.

Additionally, supporting overall horse health remains essential. Supplementing with a high-quality, water-dispersible source of natural vitamin E, such as Nano-E, helps support immune function in horses managing parasite burdens.

^*Shrubb, J. 2026. Current consensus and controversies in equine parasite control. Veterinary Record 198(12):e514-e516.